smtp: Try all entries in MX, not just the first one

Currently, we pick the first host in the MX list, and attempt delivery
there. If it fails, we just report the failure to the queue, which will
wait for some time and then try again.

This is not ideal: we should fall back to the other MXs in the list, as
the first host could be having issues for a long time, and not
attempting with the rest just delays delivery.

This patch implements the fallback, so we try all MXs before deciding to
report a failed delivery (unless, of course, an MX returned a permanent
failure).

internal/courier/smtp_test.go

@@ -87,7 +87,9 @@ func TestSMTP(t *testing.T) {
 	addr := fakeServer(t, responses)
 	host, port, _ := net.SplitHostPort(addr)
 
-	fakeMX["to"] = host
+	// Put a non-existing host first, so we check that if the first host
+	// doesn't work, we try with the rest.
+	fakeMX["to"] = []string{"nonexistinghost", host}
 	*smtpPort = port
 
 	s, tmpDir := newSMTP(t)
@@ -148,7 +150,7 @@ func TestSMTPErrors(t *testing.T) {
 		addr := fakeServer(t, rs)
 		host, port, _ := net.SplitHostPort(addr)
 
-		fakeMX["to"] = host
+		fakeMX["to"] = []string{host}
 		*smtpPort = port
 
 		s, tmpDir := newSMTP(t)